bricklink is the best place. ebay works okay, but only if you are willing to work hard to find a good deal.

Older space figs are pretty cheap right now, and you can get most of the classic factions for about $1.75 a fig (and these are by far the best minnifigs to have.)But most stores will only have like 3 of each faction, so you will need to accumulate them over many transactions, preferably also buying a bunch of greebly-bits as you do.